Naggaroth Night vs Ultramar Blue

How close are they?

ΔE 6.7, close: similar, with a visible difference side by side. Ultramar Blue is a little lighter, cooler than Naggaroth Night.

Can I use Ultramar Blue instead of Naggaroth Night?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Ultramar Blue is a little lighter, cooler than Naggaroth Night.

Can I use Naggaroth Night instead of Ultramar Blue?

The same difference applies the other way round: Naggaroth Night is a little darker, warmer.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.